It's not illegal perse but without a license file all rights are reserved to the owner per GitHub. So you could get sued and they'd (maybe) have a case. I won'

It's not illegal perse but without a license file all rights are reserved to the owner per GitHub. So you could get sued and they'd (maybe) have a case. I won't argue for a particular license, but you should have one. I personally won't touch a project without a license as I usually aim to monetize my work somehow and don't want a potential legal headache.